1. Field of the Invention
This invention relates generally to the production of high purity silicon and more particularly is directed towards a new and improved method and associated apparatus involving the reduction of HSiCL.sub.3, or the like, in a reaction chamber to obtain silicon and periodically melting the silicon for transfer through a U-shaped delivery tube to a crystal growth apparatus for replenishment of molten silicon in a crucible from which the crystal is grown.
2. Description of the Prior Art
There is a growing demand for polycrystalline silicon of high purity for uses for such products as solar cells, semi-conductors, or the like. At present the techniques and equipment used in the production of silicon for these purposes are both complex and expensive. The high cost of prior techniques and equipment is the result of a number of factors which include inefficient reaction chambers, time consuming batch processes, low volume output and high energy requirements.
Because of the complexity of the purification process semi-conductor grade silicon is very expensive. Perfect crystals of silicon can be grown by the Czochralski method in which a seed crystal is rotated while being slowly withdrawn from a crucible of molten silicon. If the temperature and rates of rotation and pulling are controlled with sufficient precision, the silicon in the crucible is converted into a single perfect crystal. At this stage the crystal can be doped, if desired, with a suitable quantity of an additive, such as boron, for example.
The semiconductor grade silicon can be obtained by various means such as by reacting technical silicon with dry hydrochloric acid gas and converted to trichlorosilane SiHCL.sub.3 which is then purified and thereafter decomposed at a temperature between 800.degree. and 1000.degree. C. The process and equipment used therein are more fully disclosed in U.S. Pat. No. 2,943,918. U.S. Pat. No. 3,012,862 also describes methods for producing elemental silicon.
Proposals which have been considered for reducing the cost of polycrystalline silicon, especially that intended for large area silicon solar cell arrays, generally fall into three major areas, the first being improved efficiency of the Siemens process, new chemical reactions which will produce a much greater yield of semi-conductor grade silicon than the Siemens process, and techniques to produce silicon which are intermediate between metallurgical grade and semi-conductor grade silicon, a solar cell grade. The improvements in the Siemens process have been mainly in process conditions, such as pressure, temperature and feed stock composition. The development of a new chemical process requires not only that the process can produce the required yield of high quality silicon, but also that the feed stocks can be produced in high volume and quality at a reasonable price. This is a long, expensive process involving high capital investment.
Other techniques have involved the deposition of silicon within a tube of silicon or fused silica. While this approach theoretically implies the possibility of low energy requirements by the use of external heaters and insulation, the need to use tubular silicon necessitates the production of such tubes which adversely affects the cost of the technique. The use of a fused silica tube has been done, but the lack of a suitable method for removing the silicon without destroying the tube has prevented this procedure from achieving major cost savings.
In both of the above systems, the deposition tubes are broken up and used to charge a crucible in a separate procedure. This, of course, results in a substantial amount of handling and limits the deposition technique to that where high density deposits are produced.
Accordingly, it is an object of the present invention to provide improvements in the method and associated apparatus for producing high grade polycrystalline material for use as solar cells or the like. Another object of the invention is to provide a method and associated apparatus for producing polycrystalline silicon and melt replenishment on a low-cost, semi-continuous basis.